In the field of prenatal care and treatment, it is well known that during the development of a fetus and throughout childbirth, it is often desirable and necessary to monitor the fetal electrocardiogram (FECG, also called fetal electrokardiogramm, or EKG). One period of an ECG is called a complex and is composed of a P wave, QRS waves and a T wave. The ECG or ECG trace is composed of several complexes that are somewhat periodic in time.
It is well known that the ECG waveform or the fetal heart rate is used to monitor the development and delivery of the fetus. An FECG can be used to alert the physician when the development or delivery is not proceeding satisfactorily, as indicated by an abnormal increase or decrease in fetal heart rate or an abnormal FECG, so that appropriate corrective measures may be taken. It is also known that historical ECG data can be used to correlate with fetal outcomes.
The FECG can be obtained non-invasively by placing electrodes on the abdominal surface area or invasively by placing electrodes inside the uterus of the mother. Two types of invasive electrodes, both of which are invasive to the mother, include a fetal scalp electrode which is also invasive to the fetus and an intrauterine catheter electrode which is non-invasive to the fetus.
In practice it is relatively difficult to non-invasively observe the FECG. This is due to the fact that the abdominal ECG being taken from the periphery of the mother consists of both the maternal electrocardiogram (MECG) and the FECG. This composite signal may also contain a relatively large amount of noise from muscle and breathing movements and other interferences. This interference can be especially strong during the delivery process.
It is well known that the placement of the ECG leads for reliable FECG sensing is a relatively difficult task. Thus, a reliable observation and detection of the FECG is made even more difficult. Further contributing to the difficulty of obtaining and reliably observing the FECG is the relative weakness of the FECG signal as compared to the strength of the MECG signal. The signal strength of the MECG can be somewhat similar or many times greater than that of the FECG. This is especially a problem when the maternal and fetal ECG QRS waves are coincident to each other. When such coincidence occurs, the MECG completely overlaps the FECG such that only the MECG signal is observable under usual circumstances.
In the prior art, leads are typically placed on the mother proximate to the fetus to obtain a composite MECG+FECG signal from the mother and the fetus. Leads may also be placed on the mother proximate the maternal heart such that a relatively pure MECG signal may be obtained. Estimates of the FECG are then made using the data obtained from the leads placed proximate the fetus and the leads placed proximate the maternal heart.
Currently there are several known techniques for enhancing the non-invasive abdominal ECG signal(s). These methods include the subtractive, adaptive filtering, orthogonal basis, linear combination, single value decomposition and MECG averaging and correlation.
The subtractive methods are the easiest techniques to implement. Typically the subtractive method is accomplished by trial and error. This heuristic method involves the subtraction of the pure MECG signal from the abdominal ECG by at least one lead at a given time. The result is then an estimate of the FECG at the particular lead. The subtractive technique, however, is limited to cases where the pure MECG is similar in shape and size as the abdominal ECG that contains the FECG. A variation of the subtractive method using first and second derivatives is described in U.S. Pat. No. 4,945,917, issued to S. Akselrod, et al, on Aug. 7, 1990.
The adaptive filtering technique is used to cancel a wide variety of interferences and distortions in signals, including speech signals and the MECG and noise components of the non-invasively- or invasively-obtained ECG signals. The adaptive filtering technique for non-invasive MECG cancellation uses at least one abdominal lead to obtain the FECG signal along with the interfering noise and MECG signal. At least three additional leads from the maternal thoracic area are used as reference inputs to an adaptive filter.
The inputs from the thoracic leads are used to replicate the interfering MECG signal obtained by the abdominal lead or leads through scale factors and this resulting estimate of the MECG of the abdominal lead is used to subtract the abdominal MECG. The output is thus an approximation of the FECG. This technique is described by Widrow, Glover, et al, "Adaptive Noise Canceling: Principals and Applications", Proc. of the IEEE, Vol. 63, No. 12, Dec. 1975, pp. 1692-1716.
The adaptive filtering technique for noise suppression has been published by D. Adam and D. Shavit, "Complete Fetal ECG Morphology Recording by Synchronized Adaptive Filtration", Med. & Biol. Eng. & Comput., Vol. 28, pp. 287-292, July 1990 and E. R. Ferrara and B. Widrow, "Fetal Electrocardiogram Enhancement by Time Sequenced Adaptive Filtering," IEEE Trans. Biomed. Eng., Vol. BME-29, No. 6, pp. 458-460, June 1982. The Adam, et al, method uses a Doppler echo-ultrasound signal to determine each of the FECG complex locations in the abdominal ECG and suppresses the noise using this information in an adaptive filter. The Ferrara, et al, technique uses a peak detect on the abdominal ECG signal to determine each FECG complex location and then uses an adaptive filtering algorithm similar to that used by Adam, et al, to improve the signal-to-noise ratio of the FECG. An example of the adaptive filtering technique is described in U.S. Pat. No. 4,781,200 issued to D. A. Baker on Nov. 1, 1988.
The orthogonal basis, linear combination and single value decomposition techniques are similar to the adaptive filtering approach for canceling the MECG by scaling at least three pure MECG signals found on the patient's periphery. These scaling coefficients are calculated through either the Gramm-Schmidt procedure, linear programming, or by single value decomposition. R. L. Longini, T. A. Reichert, J. M. C. Yu, and J. S. Cromley, "Near Orthogonal Basis Function: A Real Time Fetal ECG Technique", IEEE Trans. on Biomedical Eng., Vol. BME-24, no. 1, pp. 39-43, Jan. 1977; P. Bergveld, A. J. Kolling, and J. H. J. Peuscher, "Real Time Fetal ECG Recording", IEEE Trans. on Biomedical Eng., Vol. BME-33, no. 5, pp. 505-509, May 1986; and D. Callaerts, et al, "Comparison of SVD Methods to Extract the Fetal Electrocardiogram from Cutaneous Electrodes Signals", Med. & Biol. Eng. & Comput., Vol. 28, pp. 217-224, May 1990 have reported these techniques. The orthogonal basis and linear combination approaches have been implemented in real-time.
Finally, the MECG averaging and correlation technique has been reported by S. Abboud, G. Barkai, S. Mashiach, and D. Sadeh, "Quantification of the Fetal Electrocardiogram Using Averaging Technique", Comput. Biol. Med., Vol. 20, pp. 147-155, Feb. 16, 1990; S. Cerutti, et al, "Variability Analysis of Fetal Heart Rate Signals as Obtained from Abdominal Electrocardiographic Recordings", J. Perinat. Med., 14, pp. 445-452, 1986; J. H. Nagel, "Progresses in Fetal Monitoring by Improved Data Acquisition", IEEE Eng. Med. & Biol. Mag., pp. 9-13, September 1984; and T. F. Oostendorp, et al, "The Potential Distribution Generated by the Fetal Heart at the Maternal Abdomen", J. Perinat. Med., 14, pp. 435-444, 1986. The MECG averaging and correlation technique obtains MECG cancellation by only using the non-invasive abdominal ECG signal.
MECG averaging and correlation is accomplished by performing a peak detection on the non-invasive abdominal ECG to find the time location of each MECG R wave. The MECG average of the abdominal ECG is then calculated based on the peak detection result. After the MECG average is found, a correlation is performed between the average and the non-invasive abdominal ECG signal to determine the abdominal MECG complex time locations from which the MECG average will be subtracted.
The non-invasive FECG procedure has been in research for many years and there are a few commercially-available monitoring systems that use this procedure. The invasive scalp electrode is extensively available commercially for fetal heart rate monitoring and is commonly used in labor and delivery. Typical of the art are those monitors manufactured by Corometrics Medical Systems and Hewlett Packard. Typically, commercial monitors do not use the intrauterine catheter electrode.
The invasive scalp electrode as discussed above can detect a significant FECG signal such that the fetal heart rate may be determined. However, this electrode is invasive to the fetus also. It is desirable to have an electrode which is invasive to the mother but non-invasive to the fetus. This type of electrode may be called an intrauterine catheter electrode. However, the intrauterine catheter electrode, as compared to the scalp electrode, detects an MECG and the scalp electrode detects minimal MECG signals. N. J. Randall, et al, "Detection of the Fetal ECG during Labour by an Intrauterine Probe", J. Biomed. Eng., Vol. 10, pp. 159-164, April, 1988; and T. H. Strong, et al, "The Intrauterine Probe Electrode", Am. J. Obstet. Gynecol., 164, pp. 1233-1234, May, 1991, have developed their own intrauterine catheter electrodes and have tried to cancel the MECG using signal processing techniques such as adaptive filtering.
Other methods and systems have been devised to monitor FECG signals for prenatal observation and care and throughout childbirth. Typical of the art are those devices disclosed in the following U.S. Patents:
______________________________________ U.S. Pat. No. Inventor(s) Issue Date ______________________________________ 4,456,959 T. Hirano, et al Jun 26, 1984 4,519,396 P. Epstein, et al May 28, 1985 4,569,356 S. Kyozuka Feb 11, 1986 4,573,479 M. J. Tuccillo Mar 4, 1986 4,951,680 D. L. Kirk, et al Aug 28, 1990 4,961,428 C. L. Nikias, et al Oct 9, 1990 4,974,598 E. R. John Dec 4, 1990 ______________________________________
However, the above patents disclose methods and systems for obtaining an FECG signal using invasive techniques or which rely upon ultrasound, Doppler techniques, and other undesired techniques. In a number of the above-referenced patents, the desired output is the fetal heart rate and not the FECG trace or complex including the P wave, QRS waves and T wave.
Therefore, it is an object of this invention to provide a means for obtaining a clear and accurate FECG trace and complex, or any part thereof.
It is also an object of the present invention to provide a means for obtaining a clear and accurate FECG trace and complex in a manner which is non-invasive to the mother and the fetus.
Another object of the present invention is to provide a means whereby a clear an accurate FECG trace and complex may be obtained by a procedure which is invasive to the mother and which uses electrodes which are invasive or non-invasive to the fetus, the FECG trace and complex being processed in a similar fashion to the non-invasive abdominal ECG trace, such as suppressing the MECG, improving the signal-to-noise ratio of the FECG, and determining the fetal heart rate.
Still another object of the present invention is to provide a means whereby a clear and accurate FECG trace and complex, or any part thereof, may be monitored throughout gestation and delivery.
Yet another object of the present invention is to provide a means for displaying an accurate estimate of a clear and accurate FECG trace and complex, or any part thereof.
Another object of the present invention is to provide means whereby the ECG data acquired may be stored or otherwise processed as desired.
Still another object of the present invention is to provide a means whereby abnormalities in the FECG trace and complex may be detected and an attending physician alerted as to such abnormalities.
Still another object of the present invention is to provide a means whereby ECG data may be obtained to provide fetal heart rate determination using procedures which are non-invasive to the mother or invasive to the mother and non-invasive to the fetus.